Control valves regulate the flow of fluid through some systems, permitting zero, partial, or full flow of fluid through the valve in response to control commands. In addition to adjusting the flow of fluid through the control valve between different operational states of the control valve, it is advantageous to calibrate flow through the valve during the operational states. Adjusting the valve in this manner is commonly known as adjusting the trim of the control valve.
Calibration of the valve can change the flow setting at various states of operation, such as how much fluid flows while in the maximally-open state, or whether some fluid is still permitted to flow in a maximally-closed state. Other intermediary states can have regulated flow amounts set by the calibration. It is advantageous to know precisely how much fluid will flow through the valve during the various states of operation or positions of the control valve. Thus, calibrating, or adjusting the trim of the control valve, differs from simply adjusting the fluid flow through the valve. Typically, such trim is a small adjustment relative to the changes between operational states of the control valve.
Some control valves can regulate the flow of toxic and/or hazardous fluids. In some control valves, calibration or other fine adjustment of fluid flow through the valve requires opening the valve or valve assembly and directly manipulating some of the components. Other valves can permit manipulation through external means but without sufficient sealing to contain the fluid within. In either case, it is not preferable to expose an operator adjusting or calibrating the valve to the fluid within the valve assembly. Accordingly, it is advantageous to adjust the trim of the control valve without permitting leakage or exposure of the fluid flowing through the control valve.